1. Field of the Invention
The present invention relates to a radio communication apparatus and a radio communication method for performing media access control on the basis of a carrier sense state.
2. Description of the Related Art
In recent years, wireless local area networks (LANs) have become rapidly and widely used in environments ranging from offices to homes, as well as for hot-spot services in public places. Although standards such as IEEE802.11a, which uses a 5 GHz band, and IEEE802.11b/g, which uses a 2.4 GHz band have become mainstream, IEEE802.11e, which extends the Quality of Service (QoS) function to a medium access control (MAC) layer on IEEE802.11a/b/g has also been established as a standard. Presently, standardization activity of IEEE802.11n, which extends both the physical and MAC layers, has been advanced so as to establish effective throughput of 100 Mbps or more.
As regards IEEE802.11n, a method for extending the communication band is proposed as one approach to increase the transmission rate. Conventionally, while a transmission channel width of one channel to be used in an IEEE802.11 wireless LAN system is 20 MHz, the proposed method intends to achieve a 40 MHz band communication having a double-wide transmission channel width by using two channels of the transmission channel width of 20 MHz at the same time. For instance, according to an IEEE802 11n Working Group, “Draft Amendment to STANDARD [FOR] Information Technology-Telecommunications and information exchange between systems-Local and Metropolitan networks-Specific requirements-Part II: Wireless LAN Medium Access Control and Physical Layer specifications: Enhancements for Higher Throughput,” IEEE P802.11n™/D1.0, January 2006, IEEE802.11n uses adjacent channels each having a 20 MHz transmission channel width by bundling them, which enables 40 MHz band communication. One of the two channels is called ‘a control channel’ or ‘a primary channel’, and is utilized to exchange not only data but also control information for 20 MHz communication and basic service set (BSS) management. The other channel is called ‘an extension channel’ or ‘a secondary channel’, and used as a channel to extend the band in performing 40 MHz communication. The extension channel may be used by another system, a BSS of a wireless LAN, or more specifically, a wireless LAN using only the 20 MHz band, such as IEEE802.11/a/b/g. A data frame transmitted by using the channel width of 40 MHz could be received incorrectly at a destination terminal because of interference from other system or BSS of the wireless LAN existing on the extension channel. A throughput performance of the BSS in such a case is disclosed, for example, in Yoriko Utsunomiya, Tomoya Tandai, Tomoko Adachi, Masahiro Takagi, ‘A MAC Proposal to IEEE802.11n High Throughput WLAN for 20/40 MHz Coexistence’, IEICE Transactions on Communications, vol. J89-B, No. 2, February, 2006.
A method for switching a channel of a BSS to another channel or using only the 20 MHz band with the control channel is described, for example, in the specification of USA Pat. Appln. Publication No. 2006-0159003. Thus, if an interference level from other systems or wireless LAN existing on the extension channel is high, transmitting the frames by using only the control channel of 20 MHz could achieve less frame error rate and higher throughput than that using 40 MHz.
A terminal compliant with IEEE802.11n standards and having a 40 MHz communication capability may select either the transmission channel width of 20 MHz or the transmission channel width of 40 MHz for each frame. According to the IEEE802.11n Working Group, “Draft Amendment to STANDARD [FOR] Information Technology-Telecommunications and information exchange between systems-Local and Metropolitan networks-Specific requirements-Part II: Wireless LAN Medium Access Control and Physical Layer specifications: Enhancements for Higher Throughput,” IEEE P802.11n™/D1.0, January 2006, IEEE802.11n defines ‘Recommended Transmission Channel Width Action frame’ (hereinafter referred to as ‘a recommended channel width notification frame’) so that a terminal decides a transmission channel width. This is a frame to indicate the channel width (20 MHz or 40 MHz) which with a terminal wants other terminal to transmit. The recommended channel width may be specified for each partner terminal. A terminal to transmit the data frame may decide the transmission channel width on the basis of the recommended channel width described in a recommended channel width notification frame received from the partner terminal.
However, an algorithm to generate and transmit the recommended channel width notification frame at each terminal is not defined in IEEE802.11n. Especially, when the recommended channel width notification frame is transmitted and which channel width should be recommended.
As an example of the algorithm, a terminal may recommend the channel width which corresponds to its terminal capability by transmitting the recommended channel width notification frame. However, if the other terminals decide the transmission channel width on the basis of the recommended channel width frame transmitted in accordance with the terminal capability, under the environment in which the other system or BSS of the wireless LAN exists on the extension channel, the other terminals may transmit 40 MHz frame to the terminal even in a channel state which the terminal cannot receive the 40 MHz frame because of interference. Also, when the selection of channel width to recommend is not appropriate, the same thing would occur.
Thus, a problem, such that the frequency resource of the BSS is wasted and a transmission terminal wastes transmission power, is caused. Accordingly, to avoid such a problem, it is necessary a method by which the data reception terminal notifies of an appropriate channel width in the recommended channel width notification frame.